The starting of air breathing gas turbine engines at high altitudes presents substantial difficulties, particularly in the case of relatively small gas turbine engines. At high altitudes, the temperature of the environment is quite cold with the consequence that fuels have high viscosity, making it quite difficult to atomize the fuel sufficiently to ignite properly.
Furthermore, in small gas turbine engines, design constraints restrict the maximum diameter of the engine with the consequence that the frequently used annular combustors have a relatively small dome height, that is, the distance between the radially inner and outer walls of an annular combustor adjacent the radially extending wall or dome opposite from the combustor outlet. Small dome heights require additional injectors to achieve uniform burning to eliminate hot spots. As is well-known, in the operation of gas turbine engines, the higher the altitude, the lower the fuel flow required to maintain any given standard of operation. Consequently, at high altitudes, relatively low fuel flows are required and that in turn means a reduction in the pressure applied to the fuel to achieve the reduced flow rate. Thus, where the turbine fuel injectors are of the pressure atomization type, the lesser fuel pressure utilized at high altitude means insufficient pressure to cause the required degree of atomization necessary to achieve a start. This problem is exacerbated by the need for additional injectors in turbines having low dome heights because as the number of injectors increases, the flow through each decreases and the pressure differential across each is reduced in proportion to the reduction in fuel flow resulting in even poorer atomization.
Moreover, because of the relatively small dome height, gas velocities in the axial direction from the dome toward the combustor outlet are increased for any given volumetric flow rate to the turbine wheel of the engine. This in turn reduces the starting ability of the engine at high altitude as a result of lesser flame stability as well as lesser igniteability.
The present invention is directed to overcoming one or more of the above problems.